1. Field of the Invention
The present invention relates to a lens system that has a large aperture and is capable of close-up photographing.
2. Description of the Related Art
Recently, digital single lens reflex cameras including an image sensor, such as a charge coupled device (CCD) or complementary metal oxide semiconductor (CMOS), are becoming more popular than SLR film cameras. With an increase in users' needs and the development of image sensors, the digital single lens reflex cameras have been developed to have a high number of pixels. An interchangeable lens that is used in the digital single lens reflex cameras requires excellent optical performance.
A digital single lens reflex camera includes a reflective mirror. A reflective mirror bends the optical path 90 degrees before photographing so that a user can recognize a photographing angle through a view finder. After photographing, the reflective mirror is inclined 45 degrees so that light is not irradiated on the reflective mirror, but is irradiated in an image sensor. In regards to a digital single lens reflex camera, an interchangeable lens requires a long back focal length so that the reflective mirror does not collide with the interchangeable lens. A double Gauss lens system is widely used to satisfy the long back focal length.
By using the double Gauss lens system, spherical aberration that results from a large-aperture lens system can be efficiently reduced and a back focal length can be obtained in the digital single lens reflex cameras. In the double Gauss lens system, an aberration is corrected by using symmetry, in which two lens groups disposed at both sides of a stop have similar refractive powers.
In the double Gauss close up lens system during focusing, a front lens group moves and the back lens group is fixed. However, when a large-aperture lens system having a small F-number is used with the double Gauss close-up lens system, the diameter of the back lens group increases, and the size of a camera increases. Thus, it is not easy to miniaturize the double Gauss close-up lens system.